The present invention relates to iodine resistant precious metal catalyzed silicone rubber compositions. More particularly, the present invention relates to addition curable silicone rubber compositions containing an amount of metallic oxide effective to prevent premature deterioration of the cured rubber due to exposure to iodine and/or hydroiodic acid.
Addition curable, precious metal catalyzed silicone rubber compositions are well known in the art. Generally, these compositions comprise (a) an alkenyl-containing polydiorganosiloxane base polymer, (b) an organohydrogenpolysiloxane crosslinking agent, and (c) a precious metal or precious metal containing hydrosilation catalyst.
Bobear, U.S. Pat. No. 4,061,609, discloses that an improved work life can be imparted to addition curable silicone rubber compositions by including therein an effective amount of hydroperoxide inhibitor. Bobear further discloses that addition curable silicone rubber compositions can contain from 5 to 150 parts of filler or, more preferably, from 10 to 75 parts of filler. Included in the list of extending fillers are zinc oxide, magnesium oxide, titanium oxide, iron oxide, chromic oxide and zirconium oxide, among others. The Bobear patent, assigned to the same assignee as the present invention, is incorporated by reference into the present disclosure in its entirety.
Recently, precious metal catalyzed, addition curable silicone rubber compositions of the type disclosed by Bobear have been considered by the dairy industry as a replacement for flexible PVC and organic elastomers in milk tubing and dairy inflation applications, respectively. Potential advantages of such addition cured silicone rubber compositions include flexibility at low temperatures, inherent resistance to bacteria growth, and extended longevity.
It has been observed that TUFEL.RTM. silicone rubber tubing has deteriorated at certain dairies within a relatively short period of time, while at other dairies it has demonstrated superior service life in excess of two years.
The present applicants have discovered that the source of such early deterioration is probably due to exposure of the silicone elastomer to iodine and/or hydroiodic acid. This conclusion was based on applicants' analysis which indicated that those dairies experiencing difficulties all use a backflush rinse cycle which exposes the TUFEL.RTM. tubing to short but repetitive contact with dilute iodine sanitizing solutions.